Conventionally, ethylene and propylene are produced via steam cracking of paraffinic feedstocks comprising ethane or ethane/propane mixtures, known as gas cracking, or propane, butane, naphtha, NGL (natural gas liquids), condensates, kero, gas oil and hydrowax, known as naphtha cracking. An alternative route to ethylene and propylene is an oxygenate-to-olefin (OTO) process. Interest in OTO processes for producing ethylene and propylene is growing in view of the increasing availability of natural gas. Methane in the natural gas can be converted into, for instance, methanol or dimethylether (DME), both of which are suitable feedstocks for an OTO process.
In an OTO process, an oxygenate such as methanol or dimethylether is provided to a reaction zone of a reactor comprising a suitable conversion catalyst and is converted to ethylene and propylene. In addition to the desired ethylene and propylene, a substantial part of the oxygenate such as methanol is converted to higher hydrocarbons including C4+ olefins, paraffins and carbonaceous deposits on the catalyst. The catalyst is continuously regenerated to remove a portion of the carbonaceous deposits by methods known in the art, for example heating the catalyst with an oxygen-containing gas such as air or oxygen.
The effluent from the reactor, comprising the olefins, any unreacted oxygenates such as methanol and dimethylether and other reaction products such as water, is separated from the bulk of the catalyst, usually by one or more cyclonic separation devices. The remaining effluent may then be treated in a number of steps to provide separate component streams, including the desired olefin streams and by-product streams. Even after separation of the bulk of the catalyst, some solids, such as catalyst fines will remain in the reaction effluent stream.
In order to increase the ethylene and propylene yield of the process, a separated stream containing C4+ olefins may be recycled to the reaction zone or alternatively further cracked in a dedicated olefin cracking zone to produce further ethylene and propylene.
Following reaction in the OTO reactor, the reaction effluent stream must be cooled before being treated to provide separate component streams. Conventionally, the reaction effluent stream is cooled to around 140 to 350° C. using one or more heat exchangers, often one or more transfer line exchangers (TLEs), before being contacted with a cooled aqueous stream in a quench tower. A quench tower comprises at least one set of internals such as packing and/or trays. Packing is usually preferred due to the advantage that a more compact column may be used. In usual operation, the gaseous stream to be quenched is fed into the quench tower below the internals and one or more cooled aqueous stream is fed into the quench tower above the internals. Thus, the gaseous stream travels upwards through the quench tower and is brought into contact with the one or more cooled aqueous stream travelling downwards through the tower (counter-currently to the gaseous stream). The cooled gaseous stream is removed from the top of the quench tower. An aqueous stream containing condensed materials is removed at the bottom of the tower, cooled and recycled to be used as the cooled aqueous stream to be fed to the quench tower.
U.S. Pat. No. 6,870,072 describes such a process for recovering heat and removing solids from the reaction effluent stream in an OTO process. In U.S. Pat. No. 6,870,072, the reaction effluent stream is quenched by contacting it with a quench medium, typically a quench device, specifically a quench tower. The water cools the reactor effluent stream and removes solids. The water containing the solids is cooled and re-used as quench medium in the quench tower.
U.S. Pat. No. 7,329,790 is directed to a process for wet scrubbing and recycle of effluent contaminating catalyst particles in an OTO process. In U.S. Pat. No. 7,329,790, two scrubbing zones are used to contact the reaction effluent stream with liquid in a counter-current fashion in the presence of trays and/or packing. That is, two quench tower-type apparatuses are used in series. Water removed from the bottom of the quench towers is recycled for re-use in the quench towers.
Such continuous recycling of the aqueous streams, and the catalyst fines contained therein, will result in solids building up on the internals of the quench towers, causing blockages.
In order to overcome this, processes in the prior art have used further cyclonic separation devices in order to increase separation of solids from the reactor effluent. Alternative methods have used more trays in the quench tower rather than the preferred packing leading to decreasing effectiveness of the column per unit length. A two-stage quench tower system, in which solids are removed in a first stage is described in WO 03/104170. Such a design would significantly increase CAPEX for the OTO process.
It would be desirable to provide a simple process for the separation of solid materials, specifically catalyst fines, from the reaction effluent stream of the OTO process, avoiding blockages in the quench tower.